Abstract Hearing impairment is prevalent and has significant implications for quality of life. There are currently no available clinical cures for hereditary inner ear disease or reliable means of preventing age related hearing loss. The mouse serves as an ideal mammalian model for understanding genetic risk factors for hearing impairment and for developing therapeutic measures. Mouse models have facilitated the discovery of genes that underlie hereditary disease in humans, have made it possible to study the role of these genes in inner ear development and function, and hold great promise as models for developing treatments for hereditary inner ear disease. This grant application builds on our observations that congenital secondary hypothyroidism causes profound deafness in Pit1dw mice and thyroid supplementation during pregnancy and early neonatal life is sufficient to protect against hearing impairment. In addition, we observed that Pit1dw hypothyroid mice have delayed and/or abnormal neurogenesis and synaptogenesis. Finally, Pit1dw mutants exhibit changes in cochlear expression of genes likely to be involved in axonal guidance and synapse formation. Based on these preliminary data, we hypothesize that thyroid hormone deficiency in the neonatal period permanently impairs neurogenesis and synapse formation with inner and outer hair cells resulting in deafness that cannot be rescued by later treatment with thyroid hormone. Moreover, we suggest that adult hypothyroidism contributes to age related hearing loss. We propose to: 1) Determine whether inner hair cell function is compromised in Pit1dw mice with congenital, secondary hypothyroidism using physiological tests of exocytosis and measurement of both calcium and potassium currents, 2) Identify a novel set of thyroid hormone regulated genes that affect axonal guidance and synaptogenesis in the cochlea using candidates previously identified by gene expression profiling of Pit1dw and normal cochlea and test the functional activity of the proteins encoded by subset of these genes on cochlear neurogenesis and function. 3) Identify the time sensitive process(es) that are responsive to thyroid hormone replacement in the neonatal period but are not rescued by later thyroid hormone treatment using physiological tests, and assessment of morphological maturation and gene expression. Our investigative team has a track record for accomplishments resulting from cross-disciplinary collaboration, which has brought expertise in auditory physiology and morphology together with expertise in molecular and developmental genetics. This interdisciplinary approach focuses on defining the fundamental process of thyroid hormone stimulation of cochlear development by uncovering the target genes of thyroid hormone action in axonal guidance and synapse formation. Basic information about the molecular genetics and hormonal regulation of cochlear innervation will emerge from these studies, potentially informing efforts at improving innervation of regenerating hair cells.